anaesthesia (n.)

1721, "loss of feeling," medical Latin, from Greek anaisthēsia "want of feeling or perception, lack of sensation (to pleasure or pain)," abstract noun from an- "without" (see an- (1)) + aisthēsis "feeling" (from PIE root *au- "to perceive"). For the abstract noun ending, see -ia.

As "a procedure for the prevention of pain in surgical operations," attested from 1846. Aesthesia "capacity for feeling" is attested in English from 1853, perhaps a back-formation.